SamMyat says:

protected void btnSumbit_click(object sender, EventArgs e)

      {

          foreach (HttpPostedFile postedFile in fileUpload1.PostedFiles)

          {

              string ext = System.IO.Path.GetExtension(this.fileUpload1.PostedFile.FileName);

 

              //Change the extension to Capital Letter

 

              if (ext.ToUpper().Trim() != ".JPG" && ext.ToUpper() != ".PNG" && ext.ToUpper() != ".GIF" && ext.ToUpper() != ".JPEG" && ext.ToUpper() != ".TIF")

              {

                  Response.Write("<script Language='Javascript'> alert ('Only image file allowed')</script>");

              }

              else

              {

 

                  string filename = Path.GetFileName(postedFile.FileName);

                  string contentType = postedFile.ContentType;

                  using (Stream fs = postedFile.InputStream)

                  {

                      //var filenumber = int.Parse(Request.Files.Count);

                         for (int i = 1; i <= Request.Files.Count; i++)

                      {

                          using (BinaryReader br = new BinaryReader(fs))

                          {

                              byte[] bytes = br.ReadBytes((Int32)fs.Length);

                              string constr = WebConfigurationManager.ConnectionStrings["SqlDbConnection"].ConnectionString;

                              using (SqlConnection con = new SqlConnection(constr))

                              {

                                  string query = "insert into ImageUpload values (@File_Name, @Image_File, @File_Type)";

                                  using (SqlCommand cmd = new SqlCommand(query))

                                  {

                                      cmd.Connection = con;

                                      var name = "IMG" + "_" + System.DateTime.Now + "_" + i;

                                      cmd.Parameters.AddWithValue("@File_Name", name);

                                      cmd.Parameters.AddWithValue("@File_Type", contentType);

                                      cmd.Parameters.AddWithValue("@Image_File", bytes);

                                      con.Open();

                                      cmd.ExecuteNonQuery();

                                      con.Close();

                                  }

                                  Response.Write("<script Language='Javascript'> alert ('The Image File is Save Successfullly.')</script>");

                                  BindData();

                              }

                          }

 

                      }

                  }

 

              }

          }

      }

 replace above code with below.

protected void btnSumbit_click(object sender, EventArgs e)
{
    int i = 1;
    foreach (HttpPostedFile postedFile in fileUpload1.PostedFiles)
    {     
        string ext = System.IO.Path.GetExtension(this.fileUpload1.PostedFile.FileName);

        if (ext.ToUpper().Trim() != ".JPG" && ext.ToUpper() != ".PNG" && ext.ToUpper() != ".GIF" && ext.ToUpper() != ".JPEG" && ext.ToUpper() != ".TIF")
        {
            Response.Write("<script Language='Javascript'> alert ('Only image file allowed')</script>");
            return;
        }
        else
        {
            string filename = Path.GetFileName(postedFile.FileName);
            string contentType = postedFile.ContentType;
            using (Stream fs = postedFile.InputStream)
            {
                using (BinaryReader br = new BinaryReader(fs))
                {
                    byte[] bytes = br.ReadBytes((Int32)fs.Length);
                    string constr = WebConfigurationManager.ConnectionStrings["SqlDbConnection"].ConnectionString;
                    using (SqlConnection con = new SqlConnection(constr))
                    {
                        string query = "insert into ImageUpload values (@File_Name, @Image_File, @File_Type)";
                        using (SqlCommand cmd = new SqlCommand(query))
                        {
                            cmd.Connection = con;
                            var name = "IMG" + "_" + System.DateTime.Now + "_" + i;
                            cmd.Parameters.AddWithValue("@File_Name", name);
                            cmd.Parameters.AddWithValue("@File_Type", contentType);
                            cmd.Parameters.AddWithValue("@Image_File", bytes);
                            con.Open();
                            cmd.ExecuteNonQuery();
                            con.Close();
                        }
                        i++;
                    }
                }
            }
        }
    }
    BindData();
}

For more details refer below articles.

Upload and save (insert) multiple files to database in ASP.Net using C# and VB.Net

Upload multiple files with ASP.Net 4.5 FileUpload control in Visual Studio 2012 and 2013

Uploading Multiple Files using FileUpload Control in ASP.Net 2.0, 3.0 and 3.5